Whatever you do, don't be a douchebag about it. A lot of time time, half the reason I get so mad at Atheists (dkamm/inv3rted are two that come to mind) is because they're such jerks about it. It's hard for me to make peace with them, but I think my standpoint is "That is what you believe, this is what I believe and I'd appreciate if you respected my views and I will extend the same courtesy onto you". A lot of the time, the majority of Atheists just feel like bashing members of any organized religion and therefore, it's hard for me to respect their beliefs and them as a person, but I try.
So what I'm trying to say is, sit down and have a respectful, calm conversation with your parents about it and ask that they respect your decision. If you have any doubts though, voice those too. I think it's good when dealing with something like this to be 100% up front and honest with someone about this huge decision you are making.